A disk drive 10 operates with a transducer 18 to read and write data on a disk 12, which is mounted on a spindle motor 11 in the disk drive 10 as shown in FIG. 1. The transducer 18 is also commonly referred to as a read/write head. The disk 12, illustrated in greater detail in FIG. 2, includes a number of concentric tracks 28 that are divided into radial sectors 30 so that data can be stored according to the track and sector addresses.
The disk 12 spins about the spindle motor 11 allowing the transducer 18 to change its position from sector to sector on the same track. The transducer 18 is moved across the disk from track to track through the movement of an actuator 14 driven by a voice coil motor/driver (VCM) 16, which is connected to a servo control system/controller 24. The servo control system 24 maintains the transducer 18 in a position over the center of the track to ensure that the disk drive 10 operates properly.
The controller 24 is in turn connected to the Position Error Signal (PES) demodulator 22 which uses servo information pre-encoded as servo pattern on the tracks to generate PESs, which represent the deviation of the transducer 18 from the track center. The PESs are generated periodically during track seek and track following operations and used by the servo control system 24 to generate a corrective signal to the VCM 16, which uses the signal to correct the position of the transducer or read/write head 18.
The most commonly used method of storing servo information is the sector servo method where each track on the disk surface includes servo track information and binary data information. In this method, the servo track information is recorded at regular intervals on each track which partitions the track into radial sectors 30 as illustrated in FIG. 2. Each of these radial sectors 30 is divided into a Servo Sector where servo track information is stored and a Data Sector where data information is stored. In the Servo Sector, servo track information is encoded in magnetic patterns commonly known as servo bursts. The PES is determined using the servo burst pattern as a reference.
One problem with the sector servo method is that the position error signal (PES) is only available when the head is reading the servo sector. When the head is at the data sector, there is no method available for accurately determining that the head is on track or on track center throughout read/write operations. To ensure that the transducer is on track throughout its operation, an increase in the sampling rate of the position information would improve the performance and accuracy of the servo control system. However, an increase in the sampling rate would require either an increase in the number of servo sectors within each track or an increase in the spindle speed so that the servo sector will pass the head more often.
Unfortunately, if the number of servo sectors is increased, the amount of disk space for the storage of user data is reduced. Alternatively, if the speed of the spindle motor is increased, mechanical disturbances from the spindle motor, such as Non-Repeatable Run Out (NRRO), are also increased. Such disturbances contribute to the degrading of PES and reduce servo performance and available data storage space. Other methods were also unable to overcome having to compromising data storage space in a disk.
In the article, “A Novel Data Servo Method”, Lianna He et al, IEEE Vol. 32 No. 5 September 1996, the head position error signal can be directly extracted from the data sequences encoded by a modified Group Inter-Track Orthogonal Coding (GITOC). In this method, the original data sequences are divided into series of 2-bit groups and further coded to eight-code bits using eighth order Hadamard matrix coding rules. While this method allows continuous PES in the user data sector to facilitate precise track following, the dividing of user data into 2-bit groups and further encoding into 8-code bits limit the number of bit cell reserved for storing user data bits, hence limiting the storage space in the disk drive.
U.S. Pat. No. 6,028,731 (Bond) discloses a servomechanism which uses a structure having a track sandwiched between 2 servo tracks to provide continuous track following accuracy of the read/write head. This method provides continuous and simultaneous track following but at the expense of disk space which is used for storing the additional servo tracks.
In another article, “Customizable Coherent Servo Demodulation for Disk Drives” by Daniel Y. Abramovitch, in the IEEE paper (Vol. 3 No. 3, September 1998), an algorithm for determining position error in a servo sector is disclosed. The algorithm is based on use of mixing signals for filtering of broadband noise from servo burst patterns. This algorithm can be applied to analog, digital or hybrid forms of analog and digital signals from servo burst patterns. However, this method does not provide a continuous PES in the user data sector and therefore does not provide for continuous track following in the user sector.
In view of the limitations of existing prior art, there is therefore a need for a method that provides position information within the user data block and increases the sampling rate while avoiding an increase in mechanical disturbances. The increased sampling rate would in turn, allow a faster response from the servo controller with increased accuracy.